Flat-panel displays made in accordance with known liquid-crystal display (LCD) technologies are both limited in size and expensive to manufacture. Inexpensive, larger displays can, in principle, be made by assembling smaller display "tiles", but such a resultant, larger display generally reveals the seams between the tiles. How to conceal the seams between tiles, so that the assembled, large display looks like a continuous, one-piece unit, is very difficult, because the human eye can detect minute irregularities as long as they are arranged in patterns.
One method for making tiled displays is to connect four tiles together at common locus lines, using an adhesive sealant along the seams between adjacent tiles. The tiles are assembled in an in-plane fashion. This type of construction is shown in co-pending U.S. patent application bearing Ser. No. 08/652,032 (filed on May 21, 1996), entitled "Construction and Sealing of Tiled, Flat-Panel Displays", hereby incorporated by reference. This method of construction consists of assembling LCD tiles in a planar fashion, by locating the appropriate alignment marks disposed on each tile with matching marks on a common reference plate, usually on the cover or back plate. Specific overlay tolerances are required from such an alignment. The tiles are attached to the cover plate and back plate with an optical adhesive. Prior to assembly, the adjacent tile edges are finished such that they in their final position in the assembly provide a seamless appearance. In addition, the cover and/or back plates may contain opaque masks used to "hide" the seams. The back masks can also serve as light collimating means.
Liquid-display tiles may be fabricated in the same manner as monolithic LCDs with one exception: the inner tile edges at the inter-tile seams generally must have pixels located approximately within a pixel pitch from the inner seal edge, so that the pixel pitch across the seam is substantially the same as that on the tiles. In addition the tile edges will usually have design features that control the position of the seal material in relation to the pixel apertures and that contribute to keeping the cell gap uniform. These issues are described in co-pending patent applications Ser. No. 08/949,357 (filed Oct. 14, 1997) and Ser. No. 09/368,921 (filed Aug. 6, 1999).
As in monolithic LCDs, the two glass plates comprising the LC tile are spaced apart by small transparent balls, while the seal adhesive thickness is generally determined by spacer balls or cylinders embedded in it. Thus there will inherently be some variation in the LC layer thickness near the edges of the tiles. This variation in the LC cell gap causes noticeable discoloration of the pixels near the seams in a tiled, flat-panel LC display. No such artifacts are generally observed on monolithic LCDs because their pixels are never placed into close proximity with the seal located completely outside the pixel array.
A pixel in a color LCD is customarily made up of an aperture opening containing sub-pixels for each primary color, usually red, green and blue (RGB). Because of spatial variations in the amount of optical retardation experienced by light traveling through the LC cell, different wavelengths of light produce dissimilar visual effects, when variations in the LC cell gap thickness exist. This phenomenon compounds the problems arising in the production of tiled flat-panel liquid crystal displays that are required to exhibit a seamless appearance.
The majority of liquid-crystal display modules made today are digitally controlled devices. An optical transmission-drive voltage relationship (T-V curve) or "gamma curve" relates the digital signal values at the input to the voltage across the liquid crystal cell, and therefore to the luminance of each sub-pixel on the display. Unless otherwise noted, the T-V curve is here considered to be an effective relationship that includes the entire display system response from the electronic drive signal to the resulting luminance. In some color schemes, a linear effective relationship is desired, e.g., the NTSC R, G, B system. In other color schemes a more general weighted response curve is preferred, for example, to meet psychophysiological expectations of the viewer. Color is produced by overlaying color filter layers on top of the sub-pixel apertures. Usually three separate color filters are used to produce primary colors that through additive mixing generate all hues within the desired color gamut. In tiled, liquid crystal displays, small cell gap variations near the seams or between tiles result in changes in the effective T-V curve. As a consequence, a tiled flat panel display has an objectionable, "tiled" appearance, in spite of all efforts to geometrically hide the actual seams. Pixel areas near the seams or the tile boundaries will inevitably become visible, because their effective T-V curves differ from those of pixels in the interior of the tile.
Pixel rows and columns adjacent to the inner seams in a matrix-addressed tiled display also have a somewhat different electro-optical response compared to interior pixels away from the seams. For example, in a display with four tiles, these pixels form visually disturbing vertical and/or horizontal bands around the seams through the center of the tiled display. In particular, it is common for such a tiled display to appear seamless in white and black fields, but have quite visible seams in gray-scale fields between white and black. Since there usually are 256 gray-scale levels for each primary color, there is ample opportunity to reveal the seams, especially combined with the ability of the human eye to detect regular patterns emanating from the seams.
Another mechanism that causes color variations in tiled displays arises from the mis-registration of objects in the optical stack. Color filter layers on each tile may be mis-aligned compared to the to the thin film transistor layer. Also, the external masks on the cover and back plates may be mis-registered compared to alignment marks in the stack. Further, the lateral and height positioning of each tile compared to the color filter and thin film layers always involves alignment tolerances. As a consequence each tile edge show a slightly different response to the partially collimated light beam that is traversing the optical stack of the tiled LC display.
Additionally, the LC domains at pixels close to the inner seams are known to have different electro-optical properties. This may be caused, for example, by solvents leaching into the LC material from the seal adhesive. Furthermore, the absence of adjacent pixels, and therefore fringing electrical fields originating from them, also contributes to the dissimilar characteristics of the electro-optical properties of edge pixels.
The seam areas between tiles usually produce additional anomalous stray light that is absent on pixels away from the seams. Anomalous stray light can originate either from the backlight or ambient light, or be produced in the optical stack of the display. These effects arise either from the discontinuous tiled LCD structure or from imperfections in the manufacture or assembly of the tiled display. For example, specular and diffuse reflections from an irregular tile edge surface finish or sub-surface micro-cracks may occur. Air bubbles in the adhesive, material defects, or chipping of the tile glass edges may also contribute additional light emanating from the pixels near the edge of the tiles. Flaws in the tiles or assembly also tend to cause some depolarization of the light passing through pixels close to the seam areas, thus contributing to a brighter than normal appearance of such edge pixels, yet another optical anomaly at the tile edges. Differences in misalignment of apertures, color filters, or thin film layers among adjacent tiles produces multiple source for stray light at seams. Tile misregistration either in the lateral direction parallel to the glass surfaces (Y, Y direction) or in the perpendicular direction determined by adhesive layers provides further sources. Anomalous light at the seams induced by ambient light will be most pronounced, when the display shows a low luminance image (e.g. close to black levels) and ambient light impinging of the display is strongest.
Further, "artificial" boundaries similar to seams can be created by the electronics used to drive the LC display. These artificial boundaries are common optical artifacts that can be observed on "dual scan" notebook personal computer displays. For example, in a four tile, 2.times.2 tile array display, boundaries which may exhibit optical artifacts at the tile edges can therefore be caused by "quad scan" control electronics and LC panel drivers. In such a quad-scan drive both row (gate) and column (data) lines for each quadrant of the display are driven independently using progressive scans. When the voltages, timing, or other elements of such electronic circuits do not match exactly, artificial electronic "seams" may be created. Such electronic "seams" can, of course, also arise on monolithic LCD's which do not have any "physical" seams.
Any corrections made to images on tiled electronic color displays are subject to the luminance and color difference discrimination thresholds of the human vision system. Different threshold criteria apply to abrupt and gradual changes in luminance and chromaticity. These thresholds have been described in our U.S. patent application Ser. No. 08/593,759, filed Aug. 26, 1997, now issued as U.S. Pat. No. 5,661,531, as well as our co-pending U.S. patent applications Ser. No. 08/649,240, filed May 14, 1996; Ser. No. 08/780,911, filed Jan. 9, 1997; and Ser. No. 09/173,468 filed Jun. 30, 1999, all included herein by reference.
The present invention provides compensating means to correct for undesirable optical, electro-optical, and ambient light aberrations and for electronic anomalies, near the tile edge at the tile-to-tile seams, whether of vertical or horizontal orientation. Correction means are applied so that luminance and chromaticity of the tiles in the display appear to be essentially uniform across the seams.
It is an object of this invention to provide compensating means to correct for the effects of LC cell gap variations. Cell gaps at the tile edge rarely match those in the interior of the tile, thereby causing visible seams. Correction means are applied so that luminance and chromaticity of the display becomes visually continuous across the seams.
It is another object of this invention to provide compensating means for correcting for the effects induced by any other optical, electro-optical, mechanical, or structure related anomalies in tiled LC displays, including those due to variations in the cell gap so that the seams become visually imperceptible. Chromaticity and luminance variations at the seams are corrected or smoothed over multiple pixels so that residual variations become visually essentially uniform. Luminance and chromaticity variations are thus suppressed and seams become visually imperceptible.
It is another object of this invention to correct for optical aberrations caused by artificial boundaries (seams) due to partitions of electronic scans, control and drivers, whether occurring on tiled or monolithic display.
Another objective of this invention is the electronic correction of the brightness of all of the pixels across the interior of the tiles so that the display appears to be of uniform light intensity or luminance. Such corrections are made for each display assembly and are unique to that display.